Riko Mine Lupin IV is a female student at Tokyo Butei High School, former operative of IU, and member of the Butei team Baskerville. Despite being a Rank A in the Inquesta department, she's ironically known as the biggest idiot of her class due to her constantly sleeping during lectures. However she lives up to her rank by procuring and providing the most reliable information from the school's underground information networks in addition to keeping tabs on every student in the building.

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Riko is very short, around the same height as Aria at 147 cm (4'10"). She has long, blonde hair that falls below her waist and has twin tails styled on the side of her head. Despite having an overall appearance similar to Aria's, Riko's bust is considerably larger than Aria's (much to Aria's jealousy).

Her regular clothing is a typical Butei uniform modified to a gothic lolita style. Outside of campus, she enjoys cosplaying as maids, cats, ecetera.

Typically, she is a rather scatter-brained and playful. She loves giving nicknames to others (Kinji as Ki-kun, Reki as Rekyu, etc.). She is also one of the few people who knows Kinji's secret, resulting in her love of sexually teasing him in order to activate it. Riko also has a fondness for fancy clothing, especially that of Gothic lolita. She's also a major otaku and loves playing dating sim video games to the point where she regularly attends comikets and often puts things into dating sim terms.

During tough situations, however, she breaks this facade. Kinji calls this side of her personality "Dark Riko". With the facade removed she uses rough language, her eyes emit an intimidating glare and she refers to Aria as Holmes (French: Olmes) instead of Aria. Her typically elegant fighting style becomes vicious and her bloodlust spikes tremendously.

Born to Arsene Lupin III and (presumably) Fujiko Mine, Riko showed promise in her learning parent's craft but didn't quite inherit their full capabilities. As a result, she was often treated with care from her maids due to her newfound role of producing a Lupin who did inherit her family's talent - something she would come to resent her entire life.

Around the age of 8, however, her parents would die in a car crash and, the mysterious Vlad took her in by pretending to be her relative. He woul lock her up in an underground dungeon in order to conduct various experiments for creating an ideal 5th Lupin in the name of IU. The poor conditions of her imprisonment would greatly dampen her natural grown, causing her to remain 147 cm (4'10").

Eventually, she negotiated her freedom from Vlad's captivity and became a member of IU. As the descendant of Sherlock Holmes was making a name for herself, Riko swore vengance against her and made it her lifelong goal to slay Holmes in order to prove herself worthy of her family's name and legacy - if not superior.

Riko first makes her first apperance by doting on Kinji with his seemingly sexual relationship with the new transfer student however she is quickly silenced once Aria begins shooting up the room. She would later be hired by Kinji in order to investigate Aria's background and, following a payment of dating sims, Riko delivered an extensive profile on Aria, detailing her career back to her time as a fresh recruit at the English branch in London.

Riko would primarily be opperating behind the scenes for the early parts of the arc, however her [unbeknowst] influence was very heavily felt by Kinji and Aria. Using her ties to IU, she smuggled Jeanne d'Arc and Kyouchikytou into academy grounds and began setting the stage to draw Aria out. Using an arcade voucher, she attempted to manipulate Kinji and Aria to bond and form a partnership. It seemed everything was going as planned as they were the first Butei on the scene to respond to a busjacking she staged however the duo fell out shortly after the case so Riko had to take a more direct approach.

By feeding Kinji information that the butei killer may be hijacking the plane Aria is taking back to England, Riko finally set up the perfect scenario for Lupin and Holmes to clash once again. Riko rode her moped to the airport and snuck aboard the plane as a flight attendant, taking control of the plane before Kinji could arrive. Riko managed to get Kinji and Aria to make up and, now that the two are partners again, made an announcement in the "voice" of the butei killer that the plane has been hijacked. Kinji and Aria immediately make their way to the plane's bar and confront Riko. After revealing herself as the descendant of Arsene Lupin, a vicious fight between Aria and Riko that resulted in Riko's near capture. However Riko caught both of them offguard by revealing her ace - hair manipulation from her Irokane rosario, resulting in Kinji being disarmed and Aria being critcally injured.

Riko would ultimately allow them to flee as her "Dark Riko" personality caused her sadistic bloodlust to lapse her better judgement but she slowly followed, constantly taunting them as she made her way down the plane. She would eventually catch up and corner them in Aria's private room and, following a brief exchange with Kinji, seemingly saw through the trap they had set by unloading her gun into the closet behind her rather than the bump on the bed. Her "Dark Riko" personality once again had seemingly clouded her judgement as Aria was niether on the bed nor closet but in the overhead compartment. Caught completely off guard, Aria managed to cut off Riko's twintails and she and Kinji held Riko under arrest. Not one to come quietly, Riko used her Irokane powers and rocked the plane causing the butei to lose their footing as Riko fled.

Kinji would confront her alone and, after failing to convince him to join IU in order to be reuinted with Kana, Riko blew open a hole in the plane's cabin and jumped out of it, using her special clothes as a parachute to safely fly off into the night. Presumably the IU itself was following her as not too long after returning to IU, she would be dismissed for her failure and her rosario, and thus her Irokane abilities, taken from her.

Riko would return in her trademark flamboyant fashion by tricking Kinji into thinking Aria was calling him to her room. After successfully putting him into Hysteria mode, Aria would intervene on the two and chased Riko to the roof where the two seemed to be on the verge of finishing their previous fight. Fortunately, Hysteria Kinji settled the conflict (particuarly by flustering Aria) and Riko recruited the duo for a heist on Vlad's mansion in exchange for being a witness for Aria in her mother's trial.